This invention relates to molded pipe, such as that molded from concrete, which is cured without being fired. Such pipe, especially for low-pressure applications, is joined end-to-end by fitting the spigot end of each section in the bell end of the next and sealing the joint with a gasket. In the manufacture of such concrete pipe, roughness and unevenness and voids tend to occur at the joint faces, especially at the joint face at the bell end of the pipe, and even when such faces are well formed, their concrete surfaces are abrasive and can tend to abrade the sealing gaskets as the pipes are joined.
As is known, such pipe has a tubular wall with an enlarged bell at one end. Such bell has an inner joint face presented radially inward, which is of generally cylindrical shape, but has a slight draft taper and commonly has an outward-diverging entrance mouth of greater taper at its outer end. The joint face extends from the end face of the pipe to a shoulder at the inner end of the bell. Similarly, the spigot end of the pipe has a joint face presented radially outward, and extending from the end face of the pipe to a shoulder at the inner end of the spigot. This joint face is generally cylindrical, with a slight draft taper, but has a gasket groove formed in it, and commonly has an outward-converging guide or pilot portion at its outer end.
Such concrete pipe is commonly made by compacting concrete against an outer mold shell from the inside, and the bell-end joint face is formed by placing in the end of the shell a bell mold ring having its outer face formed to define the inward-presented bell joint face. The pipe bell is formed between that ring at the inside and the jacket at the outside. This bell mold space is accessible only from its inner end, and this makes it difficult to form the joint face against the surface of the bell mold ring, without irregularities and voids. In addition, when the bell mold ring is withdrawn from the molded bell, its relative axial movement along the joint face may produce serrations or other roughness. The spigot end of the pipe, with its radially-open gasket groove, requires a special mold made of several separable pieces. This tends less to produce joint surface roughness, but increases the complexity of the mold structure and of the molding process. In any case, the cast concrete surfaces at the joint faces may be abrasive.
The present invention facilitates molding the joint face at the bell end of the pipe, eliminates roughness and voids at the bell joint face, and provides a smooth, continuous, and well-formed joint face with a non-abrasive surface for engagement with the sealing gasket. At the spigot end, the present invention likewise provides a well-formed, non-abrasive surface, but it also serves to define and form the gasket groove, and this permits use of a simpler mold structure.
In accordance with the invention, the joint face at the bell end of the pipe section, or both that bell joint face and the spigot joint face, is lined with a preformed liner ring of plastics material, which lies tight against the joint face and preferably has its edges buried and locked in the body of the concrete which is molded against it. Each liner ring includes a generally axially extending portion which overlies and forms a facing on the molded concrete joint face. This face-forming portion is joined at its outer end to a flange which extends along the end face of the pipe and is then turned back on itself into that end face so as to extend into and be buried and locked in the body of the concrete. At the opposite end of the ring, the face-forming portion of the liner ring joins a flange which extends radially along the face of the bell or spigot shoulder for a short radial distance and then turns axially to form a generally axial collar at the edge of the ring which extends into and is buried in the body of the concrete.
In accordance with the invention, concrete pipe sections are made by mounting a preformed liner ring on the usual bell mold ring of the pipe mold, or both on that bell mold ring and on the spigot mold structure, and molding the concrete in the mold in the usual way. The concrete is compacted against the liner ring or rings instead of directly against the mold surfaces, and is molded against and about the edges of the liner ring to bury those edges in the concrete and lock the ring to the body of concrete with its face-forming portion at the joint face of the pipe.
At the spigot end, the preformed liner ring is desirably formed with the gasket groove in it. Such ring may be supported on the usual separable-part core ring and related mold structure. However, since the liner ring already contains and defines the outwardly open gasket groove, it may itself serve as the mold for such groove and may be supported in the mold structure by a suitable support and the usual groove-forming separable core ring may be omitted.
The liner rings are desirably thin-walled rings, as from about 0.030 inch to about 0.100 inch in thickness, and preferably from 0.040 inch to 0.050 inch thick. The rings are not normally intended to provide mechanical strength, but to serve merely as facings which are physically supported in place and in the desired configuration by the body of concrete against which they lie as facings and which is molded in place against them while they are supported by suitably shaped mold surfaces. When a spigot liner ring is relied on as the mold for the gasket groove, a greater wall thickness within the above-indicated range may be used to provide adequate, self-sustaining rigidity. While such range is now preferred, it is contemplated that even greater wall thicknesses may be used, up to about 0.200 inch, if desired.
The liner rings may be made of any of a wide variety of plastics materials and by any of various conventional forming processes. One desirable procedure is to make the rings of thermoplastic sheet material, by thermoforming the sheet against a suitable die under vacuum.